A source of compact ultrafast pulses, where each pulse is transform-limited with a duration approaching a single optical cycle, is in high demand for such applications as ultrafast spectroscopy, fluorescence spectroscopy, photochemistry and photophysics, coherent controlled micro-spectroscopy, multiphoton microscopy, fluorescence lifetime imaging, and non-linear biomedical imaging. White-light generation by the combination of a photonic crystal fiber (PCF) and an oscillator-type ultrafast laser is a promising technology for this source.
To generate a fiber supercontinuum pulse that is compressible to less than 20 fs, the prior art has uniformly considered the benefits, if not the necessity, of selecting a short (<1 cm) fiber length, a short (<50 fs) incident laser pulse, an adequately low pulse energy (<2 nJ) (i.e., fiber transmitting power), a particular spectral range of the supercontinuum, or combinations of the four. None of these restrictions are practically desirable due to fiber-handling difficulty, laser complexity, and low power output. It is therefore useful and surprising that a <10 fs transform-limited supercontinuum pulse without the spectral selection can be generated from a relatively long (9.1 cm) nonlinear fiber pumped by a laser with a relatively long (220 fs) pulse at a relatively high (>300 mW) fiber transmitting power, as practiced in accordance with the present invention, which is described below.
All of the foregoing restrictions are intended to improve the coherence of the supercontinuum conventionally generated by a PCF having a zero-dispersion-wavelength (ZDW) located inside the spectral range of the supercontinuum. The reason such a zero-dispersion PCF is chosen for supercontinuum generation is that, within a PCF with a spectral locus of zero dispersion, soliton dynamics are available for generating the broadest supercontinuum, and, in principle, could lead to the shortest pulse. However, in order to develop a practical and useful ˜10 fs fiber supercontinuum source, it is advantageous to perform the entire spectral broadening in a normal dispersion region of the fiber, and therefore remove all these restrictions. Südmeyer et al., in Nonlinear femtosecond pulse compression at high average power levels by use of a large-mode-area holey fiber, 28 Opt. Lett. 1951-53 (2003) used this technique to generate 33 fs pulses centered at 1030 nm. McConnell and Riis, in Ultra-short pulse compression using photonic crystal fibre, 78 Appl. Phys. B 557-63 (2004), used this technique to generate 25 fs pulses centered at 830 nm. Both of the foregoing papers are incorporated herein by reference.
To generate shorter pulse along this approach, Nishizawa et al. proposed the use of dispersion-flattened dispersion-decreased all-normal dispersion fibers (DFDD-ANDiF) (in which the ZDWs are merged in frequency to the point that the spectrum of the fiber lacks anomalous dispersion altogether within the effective wavelength range) for supercontinuum generation in Octave spanning high-quality supercontinuum generation in all-fiber system, 24 J. Opt. Soc. Am. B 1786-92 (2007), which is incorporated herein by reference. Heidt, in Pulse preserving flat-top supercontinuum generation in all-normal dispersion photonic crystal fibers, 27 J. Opt. Soc. Am. B 550-59 (2010), incorporated herein by reference, suggested that PCF-based DFDD-ANDiF should give rise to an output with a high degree of coherence. These two studies noted that if a supercontinuum pulse were to be generated from this type of fibers, then it should be recompressible to the sub-10 fs regime. These studies, however, paid no regard to the polarization properties of the fiber nor of the optical fields it supports. The spectrally flat supercontinuum of both studies, however, is not compressible in practice, because neglecting polarization properties results in a supercontinuum lacking spectral coherence and thus not subject to effective compression.
According to Tomlinson et al., Compression of optical pulses chirped by self-phase modulation in fibers, 1 J. Opt. Soc. Am., pp. 139-49 (1984), a supercontinuum dominated by SPM can only undergo high-quality pulse compression through a conventional grating (or prism) compressor if an optimal fiber length is selected to linearize the chirp of the supercontinuum. This condition is not only practically difficult, but, in the case of microscopy, where a large higher-order dispersion is associated with the microscope objective, it is impossible. It is not surprising, therefore, that attempts to compress the SPM-dominated supercontinuum by means of a grating or prism compressor have been unable to compress the pulse close to the transform-limit. Additionally, the main temporal peak of the pulse, in prior practice, has often been associated with undesirably large sidelobes.